The present invention relates to bearing devices, and more particularly to bearing devices of the preload adjustable type in which the preload to be applied to the bearing is positively and properly variable with changes in the rotation conditions and loading conditions of the bearing.
Generally preload-adjustable roller bearings are adapted to carry radial and thrust loads. To improve the accuracy of rotation and rigidity of the bearing system for supporting a rotary shaft, the bearing is subjected to an axial preload and operated in the preloaded condition in which the clearances between the constituent members of the bearing are negative.
The amount of preload to be given to such bearing must be properly determined in accordance with the kind and amount of the load acting on the bearing, the speed of rotation of the bearing, accuracy of rotation required of the shaft, etc. However even if the bearing is given a proper initial preload when mounted on the rotary shaft and installed in a housing, it becomes frequently necessary during rotation of the bearing to greatly alter the operating conditions of the machine in which the bearing is incorporated, consequently varying the rotational speed of the shaft and the load acting on the bearing and entailing thermal expansion of the shaft, housing, etc. due to heat generation. As a result, the initial preload is seldom maintained as a proper preload always during and after rotation of the bearing, and the acting preload may become excessive or insufficient, adversely affecting the rotation accuracy and performance of the bearing. Even during the rotation of bearing, furthermore, there arises the necessity of properly adjusting the preload with changes in the speed of rotation and the load on the bearing.
Accordingly in order to render the preload-adjustable roller bearing rotatable with improved accuracy and rigidity while supporting a rotary shaft, it is required to properly adjust the preload always in accordance with changes in the conditions involved during rotation.
A construction overcoming the above problem is known in which a preload-adjustable roller bearing for supporting a rotary shaft is installed in the housing of a machine portion, with the outer ring loosely fitted in the housing bore, the housing accommodating a pressing member for pressing and sliding the outer ring axially thereof and a member defining a pressure chamber having the pressing member as a movable member to apply the static pressure of a fluid to the pressing member so as to adjust the preload by altering the pressure of the chamber with changes in the speed of rotation of the bearing, bearing load and like conditions. However, because the outer ring subjected to the preload is adapted to be pushed by the fluid pressure through the pressing member which is separately made, the constituent members are likely to involve inaccuracies when they are machined and assembled. Moreover, the outer ring of the bearing loosely fitted in the housing has a usual axial dimension, namely the cylindrical outer ring surface in sliding contact with the inside surface of the housing has a short axial length, so that the outer ring inclines due to the clearance of loose fit to distort the sliding contact surfaces, consequently making it difficult or impossible to adjust the preload, or leading to uneven contact between the raceway surface of the outer ring and the rolling contact surfaces of the rolling elements. A serious trouble will then occur in the bearing. The above-mentioned inaccuracies involved in the assembling distortion of the sliding contact surfaces of the housing and the outer ring combine with irregularities in the dimensional accuracy of the constituent members, resulting in aggravated imperfections. Moreover, since the bearing device is not provided with means for preventing the rotation of the outer ring fitted in the housing, creeping takes place during rotation of the bearing, producing wear on the sliding contact surfaces of the outer ring and housing as well as on the contact surfaces of the outer ring and pressing member and impairing accurate contact between these contact surfaces. This leads to misalignment of the pressing member relative to the outer ring and/or deteriorates the perpendicular relation of the contact surfaces of the outer ring and pressing member to the axis thereof, adversely affecting the accurate rotation and preload adjustment of the bearing. When creeping becomes pronounced, the metal-to-metal sliding friction between the foregoing sliding contact surfaces and between the contact surfaces causes wear or heat generation, which reduces accuracies of the outer ring and/or the pressing member, seriously impeding the axial sliding movement of the outer ring to make it more difficult to adjust the preload.
U.S. Pat. No. 3,716,280 of R. L. Leibensperger et al. discloses a bearing device including a pressing member bearing against the large diameter end faces of tapered rollers to press the rollers axially thereof and a member defining a pressure chamber having the pressing member as a movable member for applying the static pressure of a fluid to the pressing member. However since the movable pressing member bears against the large diameter end faces of rolling tapered rollers, the accuracy of rotation of the bearing is adversely affected by a reduction in the perpendicularity, relative to the bearing axis, of the surface of the pressing member bearing against the end faces of the rollers when the pressing member slidingly moves in the axial direction and/or by a change in the position of contact between the pressing member and the large diameter end faces of the rollers due to the radial displacement of the pressing member. Moreover, such special construction is usable only in a greatly limited types of bearings. Furthermore, it is very difficult to machine the end surface of the pressing member to be contacted with the tapered rollers, since the surface must have the desired angle of contact and curvature with high precision. In addition, the pressing member needs to be inserted into an extremely confined annular space between the inner periphery of large diameter portion of the outer ring and the outer periphery of large diameter portion of a cage so as to be held in contact with the large diameter end faces of the tapered rollers. The thickness of the pressing member is consequently restricted, possibly leading to insufficient rigidity.
Thus conventional preload-adjustable bearing devices and means for varying the preload on the bearing have various drawbacks in practical uses and are not adapted for proper preload adjustment with good responsiveness in accordance with preload setting conditions such as rotation of bearing, load thereon and the like.